1. Field of the Invention
The invention relates to a screw having a shaft and a thread which is arranged in at least some areas over the circumference of the shaft and which defines a helix, and with cutting bodies of a material, hardness of which is greater than the hardness of the thread, being arranged in recesses formed in the thread.
2. Description of the Prior Art
Screws of the type mentioned above are screwed directly into a borehole that has been prepared beforehand in a substrate, with the thread of the screw cutting a complementary thread or an undercut in the substrate.
For outdoor applications, screws such as concrete screws are advantageously made of a corrosion-resistant material, e.g., corrosion-resistant steels. However, materials such as these often have an insufficient hardness for cutting into a mineral substrate so that, e.g., screw threads which are formed integral with the shaft become worn when the screw is screwed in. A hardening of the screw or of the thread is only possible under certain conditions with corrosion-resistant materials.
German Publication DE 198 52 338 A1 discloses a screw of stainless steel the shaft of which and the thread are provided with receiving boreholes which open outward for receiving cutting bodies in form of cylindrical pin-shaped cutting inserts of hardened steel which make it easier to cut into a hard, mineral substrate such as concrete or masonry. The pin-shaped cutting inserts extend longitudinally and are arranged in the receiving boreholes in such a way that their longitudinal axes extend radially outward. The pin-shaped cutting inserts are held in the receiving boreholes in the shaft formlockingly in the screw-in direction and frictionally in radial direction.
The known solution is disadvantageous in that the pin-shaped cutting inserts extend beyond the cross-sectional projection surface of the thread by the same amount, and manufacture of this screw is very complicated because the large number of pin-shaped cutting inserts must be finished individually. Further, the cylindrical pin-shaped cutting inserts project beyond the cross-sectional projection surface of the thread in such a way that the produced undercut is too large for the thread that follows, particularly in a mineral substrate because of its inhomogeneity, which decreases the load level of the screw. Further, the pin-shaped cutting inserts according to DE 198 52 338 A1 generate a high screw-in resistance which makes it difficult or even impossible to screw in the thread-tapping screw without problems, particularly with screws having a small diameter.